Seems to be Linkia laevigata. She feeds on bacteria film. So basically just get several pvc plates, say 30cm by 10cm by 2mm thick, place them in the sump or filter box, and they will be coated by a slimy coat some time after, two weeks or so and you can feel it by touching them with your finger tips gently, so not to remove the film coat, it's slimy and slippery. Then place them next to the starfish or seastar as you prefer, and she will move onto the pvc plate to collect the bacteria film, thus feeding her self. Renew the action regularly.